1. Field of the Invention
The present invention relates to a charged particle beam photolithography machine, a standard substrate for correcting a misalignment factor of the charged particle beam photolithography machine, a correcting method for the charged particle beam photolithography machine, and a method of manufacturing an electronic device.
2. Description of the Prior Art
Along with miniaturization of semiconductor devices such as LSIs in recent years, an electron beam (EB) photolithography machine is drawing attention as a leading photolithography machine that can correspond to a next-generation fine design rule. The electron beam photolithography machine is configured to deflect an electron beam by use of an electric field or a magnetic field and thereby to draw patterns on a photoresist on a wafer. It is now under consideration to introduce the electron beam photolithography machine not only to a design tool for research and development purposes but also to a “system on chip” (SoC) production line which needs to deal with limited production of diversified products.
To irradiate an electron beam onto a predetermined position with the electron beam photolithography machine, a stage for placing a wafer is moved or an amount of deflection of the electron beam is adjusted. However, in reality, it is not possible to irradiate the electron beam accurately onto a targeted position on the wafer due to distortion of the stage and the like. Accordingly, the electron beam photolithography machine is configured to irradiate the electron beam onto the targeted position while adding an amount of correction considering the distortion of the stage and the like to the amount of deflection.
There are several methods of obtaining such an amount of correction (correcting methods). One example of the methods will be described below.
FIG. 1 is a cross-sectional view showing part of an EB photolithography machine around a wafer stage according to a conventional example.
A wafer stage 1 is moved in a lateral direction of the illustration sheet by a motor 2. Then, a position of the wafer stage 1 is measured by irradiating a laser from an optical interferometer 3 onto a mirror 5 and measuring reflected light with the optical interferometer 3. Actually, there is also another motor for moving the wafer stage perpendicularly to the illustration sheet; however, the description of the other motor will be omitted herein.
Meanwhile, a reflected electron detector 4 is placed above the wafer stage 1. Reflected electrons generated when irradiating the electron beam onto a standard wafer Ws to be described later are detected by the reflected electron detector 4 configured to specify a location on the wafer Ws where the reflected electrons are generated.
To correct the electron beam photolithography machine, the standard wafer Ws for correction is firstly placed on this wafer stage 1.
FIG. 2 is a plan view of the standard wafer Ws according to the conventional example. As shown in the drawing, the standard wafer Ws includes a plurality of mark groups Ci,j in chip shapes which are arranged vertically and horizontally. Moreover, each of the mark groups Ci,j includes a plurality of marks Mp,q, which are holes formed on the standard wafer Ws and are arranged in a matrix. In this notation, (i,j) indicates the mark group on an i-th row and on a j-th column in terms of the plane of the wafer, and (p,q) indicates a mark on a p-th row and on a q-th column in each mark group.
Next, the way of correction using the standard wafer Ws will be described with reference to FIG. 3.
FIG. 3 is an enlarged plan view of the standard wafer Ws placed on the wafer stage 1.
As shown in the drawing, if there is no distortion on the wafer stage 1 and the mark Mp,q is accurately patterned on the wafer Ws, then the mark Mp,q is located in an ideal position A. However, in reality, due to an error caused upon formation of the mark Mp,q on the wafer Ws such as an error attributable to a stepper, the mark Mp,q is formed in a position B which is distant from the position A by a vector ΔP.
Moreover, the wafer Ws may be expanded or contracted if the wafer stage 1 is distorted or if the wafer stage 1 carries dust thereon. Accordingly, the mark Mp,q is further moved to a position which is distant from the position B by a vector ΔQ.
As a result, the mark Mp,q may be moved to a position C which is distant from the ideal position A by a vector ΔR (=ΔP+ΔQ).
Among these vectors, the error vector ΔP is the error caused by the stepper used for fabricating the standard wafer Ws, which is generated by distortion of reticle, for example. Accordingly, if the same stepper is used for forming a device pattern on a product wafer, this device pattern will also carry the same error ΔP. Therefore, when a resist pattern is formed by irradiating the electron beam onto the product wafer without correcting the amount of deflection, the resist pattern and the device pattern cause the same misalignment factor attributable to the wafer stage 1 in the amount equal to ΔQ, and the misalignment factor does not include ΔP.
Therefore, as shown in FIG. 4, before and after placing a subsequent product wafer W on the wafer stage 1, a point D1 corresponding to the mark Mp,q on the product wafer W is moved by ΔQ irrespective of ΔP.
Accordingly, even if the electron beam is irradiated onto a point D2 without considering the error vector ΔQ, the electron beam will not be irradiated onto a point D2 on the product wafer W on the wafer stage 1, but will be irradiated onto a point distant from the point D2 by −ΔQ, i.e., onto the point D1 on the product wafer W before being placed on the wafer stage 1.
To avoid this, conventionally, the above-described error vector ΔQ is obtained in advance, and the electron beam is further deflected by this ΔQ when irradiating the electron beam onto the product wafer W so as to irradiate the desirable electron beam onto the point D2 on the wafer stage 1.
As shown in FIG. 3, ΔQ is equal to ΔR−ΔP. Accordingly, ΔQ is obtained by measuring the ΔR and ΔP.
Among these vectors, ΔR is obtained as follows. Firstly, as shown in FIG. 1, the standard wafer Ws is actually placed on the wafer stage 1. Then, the wafer stage 1 is moved by the motor 2 while confirming the position of the wafer stage 1 with the optical interferometer 3, and the wafer stage 1 is stopped at a point where the optical interferometer 3 assumes that an optical axis is aligned with the mark Mp,q. However, due to the above-described reason, the optical axis does not always coincide with the mark Mp,q at this point because of distortion of the wafer stage 1 or dust. The stage position at this point will be indicated as R1.
Subsequently, the electron beam is irradiated onto the standard wafer Ws along the optical axis without deflection. At this time, the reflected electrons from the mark Mp,q will be captured by the reflected electron detector 4 if the optical axis actually coincides with the mark Mp,q. However, this does not occur due to the above-described reason. Accordingly, the wafer stage 1 is moved by the motor 2 to find a stage position where the reflected electron detector 4 responds. The stage position at this point will be indicated as R2. The position R2 is the position of the wafer stage 1 when the mark Mp,q actually coincides with the optical axis.
Thereafter, R1−R2 is calculated by use of the values obtained as described above, and the calculated value is equal to ΔR.
On the other hand, the standard wafer Ws is put into an optical coordinate measuring instrument, and the error ΔP (see FIG. 3) not attributable to the wafer stage 1 such as the error of the stepper is measured by this optical coordinate measuring instrument.
Then, the value of ΔQ will be obtained by calculating ΔR−ΔP with the values measured as described above.
However, the optical coordinate measuring instrument used for measuring the error ΔP not attributable to the wafer stage 1 is extremely expensive. Accordingly, a unit price for measuring one mark Mp,q is costly. As a result, expenses used for measuring all the marks on the standard wafer Ws will be enormous.